1. Field of the Invention
The present invention relates to an apparatus of shielding an electric field of a very low frequency (VLF) from the front face of a cathode ray tube (CRT) in a video appliance.
2. Description of the Prior Art
A video appliance such as a monitor using a CRT generally employs an electron gun for emitting electron beams to display images on a CRT screen. To properly position the electron beams on the screen, magnetic field generated by a deflection yoke installed on a neck portion of the CRT (not illustrated) is utilized. If an electric current flows to the deflection coil to generate such magnetic field, electric field is also generated from the deflection coil. Thus, a considerable amount of electromagnetic waves always is generated around the monitor. Accordingly, an apparatus for eliminating the electromagnetic waves has been provided in a conventional video appliance.
The circuit for eliminating electromagnetic waves in the conventional video appliance described above is illustrated in FIG. 1.
Referring to FIG. 1, the conventional circuit comprises a horizontal driving pulse generation section 70 for generating a horizontal driving pulse signal according to a horizontal synchronous signal inputted from a personal computer (PC) (not illustrated), a flyback transformer 72 for generating a high voltage according to the horizontal driving pulse signal of the horizontal driving pulse generation section 70 and supplying the high voltage to an anode of the CRT 71, an electromagnetic wave offset section 74 for waveform-shaping the negative polarity pulse signal induced in an induction coil 73 integrally wound with the flyback transformer 72 at a predetermined number of turns, and a lead wire 75 for offsetting the electromagnetic wave generated from the front face of the CRT 71.
The lead wire 75 is wound at least once along the external periphery of the CRT 71 in an open-loop state.
The horizontal driving pulse generating section 70 comprises a transistor Q for being switched on and off according to the horizontal synchronous signal inputted from the PC, and a deflection coil DY formed on the neck portion of the CRT 71 for deflecting the electron beams on the screen of the CRT 71. A capacitor C1 and a diode D1 are connected in parallel between the collector of the transistor Q and one end of the deflection coil DY, while a capacitor C2 is connected to the other end of the deflection coil DY. An anode of the CRT 71 is connected to the secondary winding of the flyback transformer 72.
The electromagnetic wave offset section 74 comprises dividing resistors R1, R2 connected to one end of the induction coil 73 for adjusting the level of the negative polarity pulse signal induced by the induction coil 73, a capacitor C3 and a resistor R3 which are connected in parallel to the other end of the resistor R1 for waveform-shaping the negative polarity pulse signal, and a capacitor C4 and a resistor R4 which are connected between the resistor R3 and the lead wire 76 for eliminating the spike-like noise of the negative polarity pulse signal.
The reference numeral C.sub.HV in the drawing denotes a capacitor.
The operation of the conventional circuit for eliminating the electromagnetic wave in a video appliance will now be explained.
If power from a power supply is applied to the primary winding of the flyback transformer 72, and a horizontal synchronous signal is inputted from the PC to the base of the transistor Q in the horizontal driving pulse generation section 70, the transistor Q drives the flyback transformer 72. In other words, if the transistor Q is turned on/off, the flyback transformer 72 induces and applies the high voltage to the anode of the CRT 71. The voltage at the primary winding of the flyback transformer 72 is also inputted to the deflection coil DY as the transistor Q is turned on. Accordingly, a deflection current flows through the deflection coil DY, and the electron beams emitted from the electron gun are deflected.
The negative polarity pulse voltage Va induced in the secondary winding of the flyback transformer 72 is level-adjusted by the dividing resistors R1, R2, as expressed by the following equation. ##EQU1##
Here, the amplitude and direct current (DC) level of the negative polarity pulse voltage Va can be level-adjusted according to the resistance values of the resistors R1, R2. The negative polarity pulse voltage Va divided by the resistors R1, R2 is waveform-shaped by the resistor R3 and capacitor C3 in the electromagnetic wave offset section 74. Subsequently, the resistor R4 and capacitor C4 eliminate the spike-like noise of the negative polarity pulse voltage Va, the waveform of which has been shaped by the resistor R3 and capacitor C3. The negative polarity pulse voltage Va is then inputted to the lead wire 75, and the lead wire 75 applies only the voltage component of the inputted negative polarity pulse voltage Va without applying the current component thereof to the front face of the CRT 71. Accordingly, the electromagnetic wave emitted through the front face of the CRT 71 is offset by the negative polarity pulse voltage of the lead wire 75, which has the same amplitude as but the opposite phase to the emitted electromagnetic wave.
The conventional circuit for eliminating the electromagnetic wave in a video appliance shields the emitted electromagnetic wave by coating the surface of the CRT 71 with a conductive material of relative high cost, or by increasing the capacitance of the smoothing capacitor C.sub.HV in the flyback transformer 73. However, such way of shielding the electromagnetic wave has caused the increase of the manufacturing cost of the CRT 71. Further, the conventional circuit has failed to completely offset the electric field noise produced in the CRT 71, thereby failing to pass the standard required by the Swedish Confederation of Professionals Employees.